Voltage-gated Na+ channelopathies are associated with multiple disorders. Mutations in neuronal Na+ channels cause epilepsy syndromes, ataxia, and autism; mutations in the cardiac Na+ channels are associated with arrhythmias, sudden infant death syndrome, conduction disorders, and cardiomyopathies. A hotspot for disease-causing mutations is the channels' C-terminal domain (CTD), which harbors a calmodulin (CaM) interaction site. Because Ca2+ is the ultimate signal of electrical activity and is often perturbed in disease states, the presence of a key Ca2+ sensor (CaM) at a hotspot for channel regulation (the CTD) provides a starting point for understanding how these channelopathies cause disease. Nevertheless, regulation of Na+ channels by Ca2+/CaM is poorly understood and information is limited by the lack of structural information and challenges to investigating channel function in native cell types. Here, we build on new structural information and novel methods to investigate how Ca2+/CaM regulates Na+ channels in the context of native cell types. Our new structural information provides background and guideposts for investigating how Ca2+-free (apo) CaM and Ca2+-loaded CaM differently affect channel function; how neuronal and cardiac channels are distinctly regulated by CaM in their native cell types, and how different CaM interacting domains within Na+ channels contribute to overall Ca2+/CaM-dependent regulation. Our novel methods of studying informative mutants in cardiomyocytes and neurons will provide an understanding of how Ca2+ affects Na+ channels and thus how Ca2+ dysregulation leads to the various Na+ channelopathies. The specific Aims addressed in this proposal are to determine: 1) How Ca2+-free apoCaM controls NaV function in neurons and cardiomyocytes; 2) How Ca2+/CaM interaction with the NaV CTD controls NaV function in neurons and cardiomyocytes; and 3) How Ca2+/CaM interaction with the NaV III-IV intracellular linker controls NaV function in neurons and cardiomyocytes.